A method which is called Czochralski method (CZ method) has heretofore widely been adopted in production of silicon single crystals. This CZ method is a method in which polycrystal silicon is melted in a crucible made of a quartz glass, a seed crystal of a silicon single crystal is dipped into this silicon melt and the seed crystal is gradually pulled up while rotating the crucible, to grow a silicon single crystal by employing a seed crystal as a nucleus. It is necessary that the single crystal produced by the above-mentioned CZ method is of high purity and is also enabled to produce a silicon wafer in high yield. As for a quartz glass crucible to be used in the production, a quartz glass crucible having a two layer structure comprising a transparent inner layer that does not contain bubbles and an opaque outer, layer that contains bubbles is generally used.
In recent years, as a silicon single crystal is made large in diameter, the time for the operation of pulling up of the single crystal becomes longer and the inner surface thereof contacts a silicon melt at a temperature of 1400° C. or higher for a long time. The inner surface thereof reacts with the silicon melt and crystallization occurs in a shallow layer of the inner surface, thereby causing the occurrence of brown cristobalite in the form of ring (hereinafter referred to as a brown ring). In the above-mentioned brown ring, there is no cristobalite layer or even if there is, it is a very thin layer, however, as the operating time elapses, the brown ring expands its size and continues to grow while fusing with each other. Finally, the central portion thereof is eroded to become an uneven glass dissolving surface. Once this glass dissolving surface emerges, dislocations are liable to be generated in a silicon single crystal, which entails inconvenience in the yield in the pulling up of a single crystal. In particular, in order to grow a silicon single crystal for producing a wafer with a large diameter of not smaller than 200 mm, it is necessary to perform the operation in the CZ method for more than 100 hours, whereby the emergence of the above-mentioned glass dissolving surface becomes prominent.
It is considered that the above-mentioned brown ring occurs by employing, as a nucleus, a minute scratch on the glass surface, a portion of crystalline residue, which is an undissolved residue of a raw material powder, a defect of the glass structure or the like. In order to reduce the number thereof, the condition of the glass surface is maintained good, melting is performed at a high temperature for a long time for reducing a crystalline residual component, or as described in Japanese Patent Nos. 2811290 and 2933404, an amorphous synthetic powder is used as a raw material powder for forming the inner surface thereof. With regard to a synthetic quartz glass made of the above-mentioned amorphous synthetic powder, there are advantages in that the content of impurities can be made extremely low and the occurrence of surface roughness or brown rings on the inner surface of a crucible in accordance with the pulling up of a silicon single crystal can be reduced. However, in the case where a transparent inner layer is composed of a synthetic quartz glass and an outer layer is composed of an opaque quartz glass made of natural quartz glass, the physical properties vary widely between the transparent inner layer and the outer layer such as the difference between the fact that one is transparent and the other is opaque and the difference between the fact that one is made of a synthetic substance and the other is made of a natural one, therefore distortion is generated at the boundary between both layers. In particular, there was a problem of deformation or peel-off of the transparent inner layer at a bent portion of a crucible to which a high heat load by a heater was applied and which was contacted with a silicon melt for a long time in some cases. In addition, a crucible comprising a transparent inner layer made of a synthetic quartz glass had a drawback in that when a polysilicon was melted, the surface of the melt was liable to vibrate compared with a crucible comprising a transparent inner layer made of natural quartz glass. This vibration was frequently observed particularly at the time from seeding to shoulder formation and in the step of pulling up a front half portion of a single crystal body at an initial stage; therefore, it took a long time to perform seeding, a crystal was disordered and melted again, that is, so-called melt back occurred, and soon, whereby the productivity was decreased in some cases. Therefore, as disclosed in JP-A-2001-348294, a crucible with a multilayer structure having an opaque intermediate layer made of a synthetic quartz glass between a transparent inner layer made of a synthetic quartz glass and an opaque bulk layer made of natural quartz glass has been proposed, however, there was a drawback in that since a large amount of expensive synthetic quartz powder was used for the crucible with a multilayer structure, the price of the quartz glass crucible was high.
The present inventors have continued intensive studies in order to overcome the above-mentioned drawbacks, and as a result, they found that the yield in the pulling up of a single crystal is deeply related to the inner surface of the bent portion of a quartz glass crucible and vibration occurring on the surface of a silicon melt is deeply related to the inner surface of the straight body portion thereof, and a high yield in the pulling up of a silicon single crystal can be achieved by forming the inner surface at least at the vicinity of the bent portion in a quartz glass crucible to be used for pulling up a silicon single crystal with a transparent layer made of a synthetic quartz glass, in addition, a problem such as deformation or peel-off of the inner layer can be solved by interposing a transparent layer made of natural quartz glass between a transparent layer made of a synthetic quartz glass and an opaque outer layer made of natural quartz glass, further, the occurrence of vibration on the surface of a silicon melt can be suppressed by forming the inner surface of the straight body portion with natural quartz glass or with a very thin synthetic quartz glass layer.
On the other hand, when the number of brown rings occurring on the inner surface of a crucible in the CZ method is reduced, the surface of a silicon melt is liable to vibrate at the time of pulling up a crystal; therefore there is a problem of deterioration of workability. As a solution thereof, it was fount that when the ratio of the number of brown rings occurring in the range from the initial surface level of a silicon melt in the pulling up of a single crystal to 0.3 M in terms of a length M from the initial surface level of the silicon melt to the surface level of the remaining melt after pulling up a single crystal measured along the inner surface of a quartz glass crucible to the number of brown rings occurring in the range up to 0.3 M from the surface level of the remaining melt after pulling up is set in a specific range or higher, vibration on the surface of the melt does not occur and the yield in the pulling up of a single crystal becomes high.
Accordingly, an object of the present invention is to provide a quartz glass crucible for pulling up a silicon single crystal in which the occurrence of vibration on the surface of a melt is suppressed, the occurrence rate of surface roughness on the inner surface of a crucible is low even in a long time operation, and a silicon silicon single crystal can be stably pulled up.
In addition, an object of the present invention is to provide a method that can produce a quartz glass crucible for pulling up a silicon single crystal having the above-mentioned excellent properties at low cost.